This study describes the context and potentially catastrophic consequences of glacier retreat near Dry Bay, Alaska within Glacier Bay National Preserve. Rapid landscape change in response to glacier retreat is generally too large scale and unpredictable to be managed, but the anticipated human impacts associated with this retreat are severe enough to necessitate attention and advance planning.The Alsek River is a glacial river that drains into the Pacific Ocean in Dry Bay, Alaska and has been a hub of human activity like fishing and travel for centuries. Upstream from Dry Bay, the Alsek River passes through Alsek Lake. One lobe of the Grand Plateau Glacier terminates in Alsek Lake (labelled GPG-AL on the map), while the other terminates in Grand Plateau Lake (GPG-GPL). In the early 1900s, the Alsek Glacier (AG) and Grand Plateau Glacier filled the basins currently occupied by the lakes. They have since retreated, producing meltwater that drains into the lakes. Between 1981 and 2019, as the ice retreated and they received more water, the lakes more than doubled in size. Currently, the Grand Plateau Glacier is all that separates Alsek Lake from Grand Plateau Lake and its drainage. This study addressed the likelihood of the lakes connecting when the glacier retreats, and, if so, whether the Alsek River will abandon its existing outlet at Dry Bay in favor of the Grand Plateau outlet to the Gulf of Alaska. The change would result in abrupt ecosystem-level changes and major socioeconomic disturbance to traditional, commercial, and recreational activities.
A team of researchers from the National Park Service, University of Alaska Fairbanks, and University of Arizona used a variety of remote sensing data collected from satellites and airplanes to determine the position and conditions of the lakes and glaciers through time. Remote sensing data provide scientists with information about large areas or long timescales that would be infeasible to collect from the ground.The terminus positions of the Alsek and Grand Plateau lobes were identified by looking at true-color Landsat images that were available from 1981 through 2019. The surface elevations of the ice were measured using aircraft-based laser altimetry systems implemented by NASA’s Operation IceBridge. This was done once in 2017 and once in 2020 so thinning rates could be calculated from the elevation change and elapsed time. In 2020, Operation IceBridge also acquired airborne radar sounding profiles that provided measurements of the glacier bed elevation and ice thickness. Ice flow fields (maps of the speed and direction of ice movement) were available from NASA’s_MEaSUREs ITS_LIVE project. River elevations and stream gradients were derived from the Alaska IfSAR elevation dataset.
Retreat rates and thinning rates are accelerating.
The Alsek and Grand Plateau Lobes of the Grand Plateau glacier have retreated over 7 and 5 kilometers, respectively, since 1981. Retreat of the Alsek Lobe accelerated in 2011-2016, including a dramatic 3 km retreat over the summers of 2014 and 2015. Retreat of the Grand Plateau Lobe was steadier, but analysis of surface elevation profiles since 2005 reveal that its thinning rate accelerated in 2014. By 2017-2020, portions of the Grand Plateau Lobe were thinning at rates up to 10 meters per year and the average thinning rate of the Alsek Lobe was closer to 2 m/yr. The difference in the behaviors of the two lobes reflects different buoyancies of the floating termini and changing directions in ice flow.
When will the lakes connect?
Ice-penetrating radar shows that the bed of the Alsek Lobe is significantly below sea level and deep enough for the lakes to connect when the Alsek Lobe retreats. The surface elevations of the two lakes are similar, providing reason to believe that they may already be hydraulically connected. Retreat rates and thinning rates are accelerating, but even based on conservative trends, the Grand Plateau Glacier will almost certainly retreat enough for the lakes to connect within the next few decades.
Will the Alsek River change its course?
Once the lakes are connected, the Alsek River has two potential outlets. Its current channel flows from Alsek Lake to Dry Bay as a braided stream composed of sand and gravel with a gradient of 1.3 m/km. The outlet from Grand Plateau Lake, however, is a confined channel with boulders and a notably steep gradient of 95.5 m/km. Despite the boulders in the Grand Plateau channel, which may limit erosion, the combination of its significantly steeper gradient and the Alsek River’s historic tendency to abandon channels indicates that the Alsek River will most likely abandon its modern outlet in favor of the Grand Plateau outlet.
What will happen if the Alsek River moves?
If the Alsek River abandons its 25 km+ course to Dry Bay, ecological repercussions would include the degradation of anadromous fish (like salmon) habitat in the abandoned channel by the introduction of fine sediment and increased vegetation. Meanwhile, the well-established human activity and developments along the Alsek River would become impossible or impractical to maintain at the abandoned channel. The modern Alsek River lies within Glacier Bay National Preserve, which protects the continuation of traditional subsistence uses, sport hunting, and commercial fishing opportunities, as well as recreational use. Importantly, none of these activities or developments to support these uses would be legally permitted at the Grand Plateau outlet because it lies within the designated wilderness of Glacier Bay National Park.
The Grand Plateau Glacier retreat and consequent Alsek River reorganization stands out among the rapidly evolving glacial landscapes along the coast of southern Alaska because of its scale, ecosystem consequences, and range of socioeconomic impacts. The range of established human activities on the lower Alsek River will undoubtably change if the main outlet shifts to Grand Plateau Lake. The sparse human population and protected status of the land limits the possible human responses to the shift. The ecosystem has adapted to previous changes in the river outlet and will respond to future changes too. The ecosystem services (intersection of natural and human processes) provided by the modern Alsek River should be evaluated before the river changes course. This baseline data will allow the National Park Service to implement adaptive management solutions as the landscape changes.
Michael G. Loso, Christopher F. Larsen, Brandon S. Tober, Michael Christoffersen, Mark Fahnestock, John W. Holt, Martin Truffer, 2021. Quo vadis, Alsek? Climate-driven glacier retreat may change the course of a major river outlet in southern Alaska. Geomorph. 384. https://doi.org/10.1016/j.geomorph.2021.107701